/**
 * Mega pixel image rendering library for iOS6 Safari
 *
 * Fixes iOS6 Safari's image file rendering issue for large size image (over mega-pixel),
 * which causes unexpected subsampling when drawing it in canvas.
 * By using this library, you can safely render the image with proper stretching.
 *
 * Copyright (c) 2012 Shinichi Tomita <shinichi.tomita@gmail.com>
 * Released under the MIT license
 */
(function() {

	/**
	 * Detect subsampling in loaded image.
	 * In iOS, larger images than 2M pixels may be subsampled in rendering.
	 */
	function detectSubsampling(img) {
		var iw = img.naturalWidth,
			ih = img.naturalHeight;
		if (iw * ih > 1024 * 1024) { // subsampling may happen over megapixel image
			var canvas = document.createElement('canvas');
			canvas.width = canvas.height = 1;
			var ctx = canvas.getContext('2d');
			ctx.drawImage(img, -iw + 1, 0);
			// subsampled image becomes half smaller in rendering size.
			// check alpha channel value to confirm image is covering edge pixel or not.
			// if alpha value is 0 image is not covering, hence subsampled.
			return ctx.getImageData(0, 0, 1, 1).data[3] === 0;
		} else {
			return false;
		}
	}

	/**
	 * Detecting vertical squash in loaded image.
	 * Fixes a bug which squash image vertically while drawing into canvas for some images.
	 */
	function detectVerticalSquash(img, iw, ih) {
		var canvas = document.createElement('canvas');
		canvas.width = 1;
		canvas.height = ih;
		var ctx = canvas.getContext('2d');
		ctx.drawImage(img, 0, 0);
		var data = ctx.getImageData(0, 0, 1, ih).data;
		// search image edge pixel position in case it is squashed vertically.
		var sy = 0;
		var ey = ih;
		var py = ih;
		while (py > sy) {
			var alpha = data[(py - 1) * 4 + 3];
			if (alpha === 0) {
				ey = py;
			} else {
				sy = py;
			}
			py = (ey + sy) >> 1;
		}
		var ratio = (py / ih);
		return (ratio === 0) ? 1 : ratio;
	}

	/**
	 * Rendering image element (with resizing) and get its data URL
	 */
	function renderImageToDataURL(img, options, doSquash) {
		var canvas = document.createElement('canvas');
		renderImageToCanvas(img, canvas, options, doSquash);
		return canvas.toDataURL("image/jpeg", options.quality || 0.8);
	}

	/**
	 * Rendering image element (with resizing) into the canvas element
	 */
	function renderImageToCanvas(img, canvas, options, doSquash) {
		var iw = img.naturalWidth,
			ih = img.naturalHeight;
		var width = options.width,
			height = options.height;
		var ctx = canvas.getContext('2d');
		ctx.save();
		transformCoordinate(canvas, ctx, width, height, options.orientation);
		var subsampled = detectSubsampling(img);
		if (subsampled) {
			iw /= 2;
			ih /= 2;
		}
		var d = 1024; // size of tiling canvas
		var tmpCanvas = document.createElement('canvas');
		tmpCanvas.width = tmpCanvas.height = d;
		var tmpCtx = tmpCanvas.getContext('2d');
		var vertSquashRatio = doSquash ? detectVerticalSquash(img, iw, ih) : 1;
		var dw = Math.ceil(d * width / iw);
		var dh = Math.ceil(d * height / ih / vertSquashRatio);
		var sy = 0;
		var dy = 0;
		while (sy < ih) {
			var sx = 0;
			var dx = 0;
			while (sx < iw) {
				tmpCtx.clearRect(0, 0, d, d);
				tmpCtx.drawImage(img, -sx, -sy);
				ctx.drawImage(tmpCanvas, 0, 0, d, d, dx, dy, dw, dh);
				sx += d;
				dx += dw;
			}
			sy += d;
			dy += dh;
		}
		ctx.restore();
		tmpCanvas = tmpCtx = null;
	}

	/**
	 * Transform canvas coordination according to specified frame size and orientation
	 * Orientation value is from EXIF tag
	 */
	function transformCoordinate(canvas, ctx, width, height, orientation) {
		switch (orientation) {
			case 5:
			case 6:
			case 7:
			case 8:
				canvas.width = height;
				canvas.height = width;
				break;
			default:
				canvas.width = width;
				canvas.height = height;
		}
		switch (orientation) {
			case 2:
				// horizontal flip
				ctx.translate(width, 0);
				ctx.scale(-1, 1);
				break;
			case 3:
				// 180 rotate left
				ctx.translate(width, height);
				ctx.rotate(Math.PI);
				break;
			case 4:
				// vertical flip
				ctx.translate(0, height);
				ctx.scale(1, -1);
				break;
			case 5:
				// vertical flip + 90 rotate right
				ctx.rotate(0.5 * Math.PI);
				ctx.scale(1, -1);
				break;
			case 6:
				// 90 rotate right
				ctx.rotate(0.5 * Math.PI);
				ctx.translate(0, -height);
				break;
			case 7:
				// horizontal flip + 90 rotate right
				ctx.rotate(0.5 * Math.PI);
				ctx.translate(width, -height);
				ctx.scale(-1, 1);
				break;
			case 8:
				// 90 rotate left
				ctx.rotate(-0.5 * Math.PI);
				ctx.translate(-width, 0);
				break;
			default:
				break;
		}
	}


	/**
	 * MegaPixImage class
	 */
	function MegaPixImage(srcImage) {
		if (window.Blob && srcImage instanceof Blob) {
			var img = new Image();
			var URL = window.URL && window.URL.createObjectURL ? window.URL :
				window.webkitURL && window.webkitURL.createObjectURL ? window.webkitURL :
				null;
			if (!URL) {
				throw Error("No createObjectURL function found to create blob url");
			}
			img.src = URL.createObjectURL(srcImage);
			this.blob = srcImage;
			srcImage = img;
		}
		if (!srcImage.naturalWidth && !srcImage.naturalHeight) {
			var _this = this;
			srcImage.onload = function() {
				var listeners = _this.imageLoadListeners;
				if (listeners) {
					_this.imageLoadListeners = null;
					for (var i = 0, len = listeners.length; i < len; i++) {
						listeners[i]();
					}
				}
			};
			this.imageLoadListeners = [];
		}
		this.srcImage = srcImage;
	}

	/**
	 * Rendering megapix image into specified target element
	 */
	MegaPixImage.prototype.render = function(target, options) {
		if (this.imageLoadListeners) {
			var _this = this;
			this.imageLoadListeners.push(function() {
				_this.render(target, options)
			});
			return;
		}
		options = options || {};
		var imgWidth = this.srcImage.naturalWidth,
			imgHeight = this.srcImage.naturalHeight,
			width = options.width,
			height = options.height,
			maxWidth = options.maxWidth,
			maxHeight = options.maxHeight,
			doSquash = !this.blob || this.blob.type === 'image / jpeg';
		if (width && !height) {
			height = (imgHeight * width / imgWidth) << 0;
		} else if (height && !width) {
			width = (imgWidth * height / imgHeight) << 0;
		} else {
			width = imgWidth;
			height = imgHeight;
		}
		if (maxWidth && width > maxWidth) {
			width = maxWidth;
			height = (imgHeight * width / imgWidth) << 0;
		}
		if (maxHeight && height > maxHeight) {
			height = maxHeight;
			width = (imgWidth * height / imgHeight) << 0;
		}
		var opt = {
			width: width,
			height: height
		};
		for (var k in options) opt[k] = options[k];

		var tagName = target.tagName.toLowerCase();
		if (tagName === 'img') {
			target.src = renderImageToDataURL(this.srcImage, opt, doSquash);
		} else if (tagName === 'canvas') {
			renderImageToCanvas(this.srcImage, target, opt, doSquash);
		}
		if (typeof this.onrender === ' function') {
			this.onrender(target);
		}
	};

	/**
	 * Export class to global
	 */
	if (typeof define === ' function' && define.amd) {
		define([], function() {
			return MegaPixImage;
		}); // for AMD loader
	} else {
		this.MegaPixImage = MegaPixImage;
	}

})();